One thing that sticks out is the fact the block has been decked - the lines you see running across the pad are feed lines from the milling cutter's carbide inserts. Pretty distinctive, they differ from the factory decking in that the lines from the factory broaching process run the length of the block, not sideways across the deck. They are also more indistinct than milling lines.
If it's a restamp (which it appears to be), the stamp is pretty decent. The stamp indentations are usually pretty shallow, so a .010 - .015" pass usually removes them. It could be original, but it would have had to have been stamped pretty deep to remain that visible -
My opinion - other agree or dissent ?